U.S. patent number 6,135,831 [Application Number 09/425,824] was granted by the patent office on 2000-10-24 for impeller for marine waterjet propulsion apparatus.
This patent grant is currently assigned to Bird-Johnson Company. Invention is credited to Neal A. Brown, Francesco Lanni.
United States Patent |
6,135,831 |
Lanni , et al. |
October 24, 2000 |
Impeller for marine waterjet propulsion apparatus
Abstract
An impeller for a marine waterjet propulsion system has blades
that are configured to reduce cavitation, vibration, noise and
physical damage to the major components of the propulsion system or
host vessel of installation. The leading edge of each blade of the
impeller is skewed forwardly over at least the outer 70% of its
span, the forward skew being maximum at the tip and being not less
than 35.degree.. The impeller has a blade area ratio of not less
than 1.5. The chord lengths of each blade increase progressively
from the point of minimum skew to the tip, resulting in reduced
loading in the cavitation critical region. A partial or full tip
band may be affixed to the blade tips.
Inventors: |
Lanni; Francesco (Walpole,
MA), Brown; Neal A. (Lexington, MA) |
Assignee: |
Bird-Johnson Company (Walpole,
MA)
|
Family
ID: |
23688182 |
Appl.
No.: |
09/425,824 |
Filed: |
October 22, 1999 |
Current U.S.
Class: |
440/49; 416/238;
440/67; 440/79 |
Current CPC
Class: |
B63H
11/08 (20130101); F04D 29/242 (20130101); F04D
29/2277 (20130101); B63H 2011/081 (20130101) |
Current International
Class: |
B63H
11/08 (20060101); B63H 11/00 (20060101); F04D
29/22 (20060101); F04D 29/24 (20060101); F04D
29/18 (20060101); B63H 001/14 () |
Field of
Search: |
;440/49,66,67,79
;416/179,223,228,238,242 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Sotelo; Jesus D.
Attorney, Agent or Firm: Baker Botts L.L.P.
Claims
What is claimed is:
1. An impeller for marine waterjet propulsion apparatus comprising
a plurality of blades, each blade having a leading edge with a
forwardly skewed region extending inwardly from the outer tip along
not less than 70% of the span of the leading edge of the blade.
2. The impeller as claimed in claim 1 and in which the skew of the
forwardly skewed region of each blade is greatest at the outer tip,
which has a skew of not less than 35.degree..
3. The impeller as claimed in claim 1 and in which the skew of the
forwardly skewed region of each blade is greatest at the outer tip,
which has a skew of not less than 50.degree..
4. The impeller as claimed in claim 1 and in which the chord
lengths of each blade increase progressively in a direction from
the root to the tip in a portion of the span corresponding to the
forwardly skewed region, the blade sections being extended to
accommodate leading edge skew.
5. The impeller as claimed in claim 1 and in which the projected
blade area ratio is not less than 1.5.
6. The impeller as claimed in claim 1 and in which the maximum
thickness of the blade section and the radius of the leading edge
both increase progressively in a direction from the root to the tip
in a portion of the span corresponding to the forwardly skewed
region.
7. An impeller for marine waterjet propulsion apparatus comprising
a plurality of blades, each blade having a leading edge with a
forwardly skewed region extending inwardly from the tip along not
less than 70% of the span of the leading edge of the blade, the
skew of the forwardly skewed region of each blade being greatest at
the outer tip, which has a skew of not less that 35.degree., and
the chord lengths of each blade increasing progressively along not
less than the outer 70% of the span in a direction from the root to
the tip.
8. The impeller as claimed in claim 7 and in which the skew at the
outer tip is not less than 50.degree..
9. The impeller as claimed in claim 7 and in which the projected
blade area ratio is not less than 1.5.
10. The impeller as claimed in claim 7 and in which the maximum
thicknesses of the blade sections of each blade and the radii of
the leading edge of each blade both increase progressively along
not less than the outer 70% of the span in a direction from the
root to the tip.
11. An impeller for marine waterjet propulsion apparatus comprising
a plurality of blades, each blade having a leading edge with a
forwardly skewed region extending from the outer tip towards the
inner root along not less than 70% of the span of the leading edge
of the blade, the skew of the forwardly skewed region of each blade
being greatest at the outer tip, which has a skew of not less that
35.degree., and the projected blade area ratio of the impeller is
not less than 1.5.
12. The impeller as claimed in claim 1 and further comprising a
circumferential tip band affixed to at least portions of the tips
of the blades.
13. The impeller as claimed in claim 12 wherein the tip band
extends along only part of the axial extents of the blades.
14. The impeller as claimed in claim 12 wherein the tip band
extends along the entirety of the axial extents of the blades.
Description
BACKGROUND OF THE INVENTION
Universally, marine waterjet propulsion systems consist of rotating
and stationary rows of blades. The rotating blade rows are termed
impellers. The purpose of the rotating blade rows is to raise the
total energy of the water passing through the propulsion system,
which can be used to produce useful thrust to propel the host
vessel through the water. The stationary blade rows are termed
diffusers or guide vanes. One purpose of the stationary blade rows,
if positioned downstream from a rotating blade row, is to recover
rotational flow energy produced by the impeller that can be used to
augment the ability of the propulsion system in producing useful
thrust to propel the host vessel through the water. If the
stationary blade row is positioned upstream from a rotating blade
row, one of its purposes is to mitigate large-scale fluctuations in
flow velocity magnitude and direction seen by a rotating blade
row.
Fluctuations in the magnitude and direction of flow velocities can
result in detrimental consequences to the performance of the
propulsion system. Specifically, they can cause fluctuating
pressures to occur on any effected blade row, rotating or
stationary. These fluctuations typically result in reduced
efficiency for the propulsion system, increased vibration, and
increased noise. In the event of severe fluctuations, the resulting
surface pressures on blade rows may be reduced to levels below the
vapor pressure of water, causing the water to boil. That phenomenon
is termed cavitation. Bubbles of water vapor are created on the
surfaces of the blades, which may coalesce into large cavities that
remain attached to the blades or which may be shed from the blade
row surfaces to travel downstream. The cavities and bubbles are
detrimental to the performance of the propulsion system in a number
of ways.
If the extent of cavitation in the propulsion system is severe
enough, blockage of the flow of water through the system occurs,
resulting in cavitation breakdown, or thrust breakdown, where the
useful thrust of the system is reduced catastrophically.
If the cavities or bubbles of water vapor find their way to a
region of the system where pressures in the flow field are above
the vapor pressure of water, rapid condensation of the water vapor
occurs as the bubble implodes back into a liquid state. The action
of that implosion is violent, resulting in excessive transient
pressure fluctuations that are forceful enough to physically damage
the structure of the propulsion system components.
Implosion of the water vapor bubbles will, at a minimum, result in
volume and pressure fluctuations sufficient to generate noise.
Additionally, these fluctuations result in vibration of the
propulsion system structure as well as of the associated host
vessel structure. The vibration can lead to fatigue failure of the
structures, as well as their radiation of additional noise both
within the host vessel and into the water.
Previously known waterjet propulsion systems suffer universally
from cavitation. Typical installations of waterjet propulsion
systems are such that fluctuations in the magnitude and direction
of flow velocities cannot be sufficiently mitigated by any
practical means without severely restricting the operating range of
the host vessel. Over some range of vessel operating conditions,
cavitation is severe enough to result in physical damage to the
propulsion system or, at a minimum, in a significant increase in
vibration and noise.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a waterjet
propulsion system which has two blade rows, one rotating and one
stationary, that for any given size exhibits greater resistance to
the onset of cavitation resulting from spatial and temporal
fluctuations in the magnitudes and direction of flow velocity than
previously known waterjet propulsion systems. Another object, which
is of particular interest for oceanographic research, recreational,
and military vessels, is to extend the operating range of the
vessel over which cavitation induced noise and vibration are not
present.
The foregoing objects are attained, in accordance with the present
invention, by an impeller for a waterjet propulsion apparatus
having an impeller with a plurality of blades, each of which is
significantly skewed or swept forwardly, that is, in a direction
opposite that of the impinging upstream water flow, measured
relative to a coordinate system fixed to and revolving with the
blade. In this way the blade tip leads the more inward portions of
the leading edge in the direction of rotation. The forwardly skewed
region extends inwardly from the outer tip along not less than 70%
of the leading edge span of the blade. The projected skew angle
measured between a line through the center of rotation and the
leading edge point of minimum skew and a line through the leading
edge point of minimum skew and the leading edge point of maximum
skew (at the blade tip) is greater than 35.degree., and preferably
greater than 50.degree..
The leading edge of the blade near the tip is typically the
location of earliest cavitation onset due to fluctuations in
upstream flow velocity related incidence. The effect of maximum
forward skew at the tip acts to introduce three-dimensional flow
affects that result in reductions of peak blade surface pressure
fluctuations, which cause cavitation to occur. The reductions in
fluctuations of peak blade surface pressures also reduce the blade
loading fluctuations and resultant vibrations, this reducing a
cause of structural damage to components of the propulsion system
due to fatigue.
It is advantageous to introduce forward skew to the blade at the
leading edge without correspondingly altering the shape of the
trailing edge. In that way, the chord lengths of the blade are made
to increase in at least the outer 70% of the blade span, resulting
in an increase in projected blade area ratio. For a specified blade
load an increase in blade area results in a reduction of the
magnitude of the induced pressure on one side of the blade. The
danger of cavitation with addition of fluctuations is thereby
reduced. In particular, the projected blade area ratio is in excess
of 1.5, the projected blade area ratio being defined as the number
of blades multiplied by the blade area projected onto a plane
perpendicular to the axis of rotation divided by the area of the
projected outline of all the blades onto a plane perpendicular to
the axis of rotation. The relatively large projected blade area
ratio results in overall lower blade surface pressure magnitudes in
the absence of flow velocity magnitude and direction fluctuations.
In the presence of these fluctuations the tendency of peak blade
surface pressure fluctuations to reach vapor pressure, and thus
cause cavitation, is commensurately reduced.
The cavitation-inducing reduction of pressure near the leading edge
of a blade which results from flow velocity incidence fluctuations
is found to be mitigated by increased nose radius of the blade
sections and by forward skew or sweep of the leading edge toward
the blade tip. The benefit of blade section nose radius, however,
is limited by the increased pressure reduction in the mean flow at
the "shoulders" of the section noses. Further, given a blade
section geometric parent form, increased nose radius implies
increased blade section thickness which can yield unacceptable
blockage and cavitation in the mean flow between the blades.
By reducing the pressure response to velocity fluctuation changes
of incidence, the outwardly forward skewed blade leading edge more
than compensates for the less than desired nose radii available to
the designer. For a given incidence fluctuation, ambient pressure
and velocity, the compensating leading edge skew angle is found to
be inversely proportional to the square root of the section nose
radius.
Another embodiment of the current invention includes a
circumferential tip band structure surrounding and fixed to the
tips of the impeller blades, which serves to prevent cavitation in
a tip gap clearance and lends improved structural integrity to the
bladed impeller assembly. The tip band may be partial or
full--i.e., it may extend along only part of or along the entirety
of the axial extents of the blades.
DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the
advantages thereof, reference may be made to the following written
description of exemplary embodiments, taken in conjunction with the
accompanying drawings.
FIG. 1 is a schematic exploded pictorial view of a marine waterjet
propulsion system assembly, which includes an embodiment of an
impeller with forward skew applied to the leading edge of each
blade according to the present invention;
FIG. 2 is a side cross-sectional view of the marine waterjet
propulsion system assembly of FIG. 1, showing it assembled and the
view being taken along the lines 2--2 of FIG. 3;
FIG. 3 is a front elevational view of the assembly of FIGS. 1 and
2;
FIG. 4 is a pictorial view of the impeller of the assembly of FIGS.
1 to 3;
FIG. 5 is a front elevational view of the impeller of FIG. 4;
FIG. 6 is a side elevational view of the impeller of FIGS. 4 and
5;
FIG. 7 is a diagram of one of the blades of the impeller of FIGS. 4
to 6, in which the blade sections are projected onto a plane
perpendicular to the axis of rotation of the impeller;
FIG. 8 is a diagram showing the peripheral contour of a single
blade
projected onto a plane perpendicular to the axis of rotation of the
impeller; and
FIG. 9 is diagram showing the combined contours of the roots and
tips of all of the blades projected onto a plane perpendicular to
the axis of rotation of the impeller.
DESCRIPTION OF THE EMBODIMENT
A marine waterjet propulsion system 20 utilizing one stationary
blade row and one rotating blade row is shown generally
schematically in FIGS. 1 to 3. The rotating blade row or impeller
22 is used in conjunction with the stationary blade row or diffuser
24 to impart energy to the flow of water through the propulsion
system, which can be used to generate useful thrust. The remaining
components of the marine waterjet propulsion system depicted, which
include an impeller housing 26, a diffuser hub cone 28, and an exit
nozzle 30, are used to contain and direct the flow of water through
the propulsion system.
Water flow enters the impeller housing 26 and is acted upon by the
impeller 22. The impeller 22 increases the energy of the water flow
through the propulsion system, which can be used to generate useful
thrust. In addition, the impeller 22 imparts rotational energy to
the water flow, which cannot be used to generate useful thrust. The
flow continues through the propulsion system to the diffuser 24
where the rotational energy imparted by the impeller 22 can be
transformed into energy which can in turn be used to generate
useful thrust. The cone 28 and the exit nozzle 30 in concert
transform the energy imparted to the water flow through the
propulsion system by action of the impeller and diffuser into
useful thrust.
The impeller 22 has a hub 40, which is shaped somewhat like
one-half of a football and has a axial hole 42 that receives a
drive shaft (not shown), to which the impeller is affixed. Six
identical, equally circumferentially spaced-apart impeller blades
44 extend in a row around the hub 40. The blade tips are in close
running clearance with the inner surface of the impeller housing
26. Forward skew is applied to the leading edge of each of the six
blades 44 of the impeller 22, as described below.
Although the embodiment of the impeller 22 shown in the drawings
and described herein is a mixed flow type of impeller, the present
invention may be applied to many different designs of impellers,
including inducer types, axial types, and centrifugal types, and to
impellers with various numbers of blades.
FIG. 7 shows a single impeller blade 44 diagrammatically. The
twenty double line curves C1, C2, etc. are projections onto a plane
perpendicular to the axis A of the impeller 22 of blade sections
formed by intersections of the blade by twenty equally spaced apart
imaginary doubly curved cutting surfaces, each of which is
generated by revolving a flow line of the flow path of water
through the passage between the outer surface of the hub 40 and the
inner surface of the impeller housing 26 about the axis A. The root
blade section C1 and the tip blade section C20 depict the extent of
the blade in the span wise direction, while the leading edge
periphery 52 and the trailing edge periphery 54 depict the extent
of the blade in the chord wise direction. The minimum projected
skew line SL.sub.min is drawn through the axis of rotation A of the
impeller and the point of minimum projected skew SP.sub.min. The
maximum projected skew line SL.sub.max is drawn through the point
of minimum projected skew SP.sub.min and the tip leading edge point
56. The projected skew angle .alpha. between the maximum projected
skew line SL.sub.max and the minimum projected skew line SL.sub.min
is greater than 35.degree. and, preferably, greater than
50.degree.. Forward skew is maintained along the portion of the
leading edge of the blade between the tip leading edge point 56 and
the point of minimum projected skew SP.sub.min, the amount of skew
diminishing progressively from the point 56 along that portion. The
point of minimum projected skew SP.sub.min is located at a distance
from the tip that is not less than 70% of the projected span of the
blade.
FIG. 8 depicts a true orthogonal projection of the area enclosed by
the periphery of a single impeller blade 44 onto a plane
perpendicular to the axis of rotation A. FIG. 9 depicts a true
orthogonal projection of the area enclosed by the combined
periphery of all six blades 44 of the impeller 26 onto a plane
perpendicular to the axis of rotation A. The projected blade area
ratio, which is calculated by multiplying the number of blades of
the impeller by the projected area of a single blade (FIG. 8) and
dividing by the area enclosed by the combined periphery of all six
blades (FIG. 9), is greater than 1.5.
* * * * *